Standard replies to 3 space extension invasion

The move Black will choose to respond will greatly depend on the circumstances, but mostly these moves reflect the following intentions:

a and b attempt to connect the stones, at the expense of a ponnuki for White or in any case sacrificing that stone in the process.

c and d show an intention to attack the stone, by threatening to engulf it. Technically c is superior (see later).

e is more a defensive move and shows that Black is not in his own sphere of influence and is prepared to let White have it her way.

f is a territorial, endgame-ish move

We will have a look at typical sequences fired off by these moves. However, the practical applications will nearly always involve neighbouring stones, which greatly affect the events and therefore also the appropriate choice of move.

Attaching underneath

Cut

First of all, if Black is not intending to crosscut with after , he should not play in the first place, unless these are endgame tactics. We are thinking of this position as a middle game invasion, so cut and connect are the governing principles.

Black sacrifices to get outside influence, while White gets some territory. Next, White can move out at a (considered correct) or b (unusual), or leave that choice to Black by capturing at c immediately (mild). The cut at d is unreasonable. Again, the presence of other stones may justify any choice.

Attaching on top

Ponnuki

Attaching on top is technically the same idea, sacrificing a stone, but obviously with the opposite result. Black connects underneath, while White makes a ponnuki. This sequence can be played when the connection is vital for Black and the resulting influence for White is acceptable, e.g. overlapping with already existing influence.

Diagonal attack

Diagonal attack

This shows an attacking mood, which must be justified by the presence of other black stones, such as e.g. at the circled point, where the formation represents a low enclosure low extension invasion. is appropriate, though contrary to basic instinct. We can now envision Black playing a as basic instinct and White jumping out at b.

Why the diagonal response

The basic instinct of is unreasonable (barring unusual conditions). puts white on the spot: now she has to play an empty triangle with . Imagine a stone already awaiting her at a: the invasion will fail.

Why the diagonal response

This is stylish, but Black cuts nevertheless and White will at best live very small. It is not said this line is impossible though: both must take it into account.

symplicity: Won't it often be better for white to capture the stone? Black has some followups like sacrificing two and squeezing, but white escapes any major attack and has gained some rightward strength.

Empty triangle again

can also be played here. White remains with the burden of creating an empty triangle or be cut at a.

Weaker attack

Keima

The keima is also a typical move for attacking, but Black does not seem to take the advantage he could in this position. Next, a to d are all conceivable. If Black already has a stone at c that again has a major effect.

Angle play

Angle play

The exchange - seems to confirm White's intent of separating the stones. If this is Black's sphere of influence, then White may play more lightly, allowing the meager connection underneath. If this is White's sphere of influence, then this move can be considered big endgame.

One space jump

Jump

Finally, shows a defensive intention, so probably Black is not in a position to attack here. If White jumps too, crossing under with a remains possible for Black, so maybe White should play b, assuming this is actually her sphere of influence.

Separated

We recognize the earlier pattern which Black can use to separate the White stone . This is rarely a desirable result for White but now the cut at a may work.